A library-orientated approach is used to gain understanding of the interactions of well-defined nanoparticles with primary human endothelial cells, which are a key component of the vasculature. Fifteen sequentially modified gold nanoparticles (AuNPs) based on three different core sizes (18, 35, 65 nm) and five polymeric coatings were prepared. The synthetic methodology ensured homogeneity across each series of particles to allow sequential investigation of the chemical features on cellular interactions. The toxicity of these nanoparticles, their uptake behavior in primary human dermal microvascular endothelial cells (HDMECs), and quantification of uptake were all investigated. The results of our studies indicated that high concentrations of gold nanoparticles (250 mu g/mL) were nontoxic and that the number of internalized nanoparticles was related to nanoparticle size and surface chemistry. In summary, the positive-charged ethanediamine-coated AuNPs were internalized to a greater extent than the negative- or neutral-charged AuNPs. Moreover, differences in the amounts of internalized AuNPs could be shown for the three neutral-charged AuNPs, whereas the uptake of hydroxypropylamine-coated particles was preferred compared with glucosamine-coated or PEGylated AuNPs. Hydroxypropylamine-coated AuNPs were found to be the most efficient neutral-charged particles in overcoming the endothelial cell barrier and entering the cell.